Numerous systems have been devised to combine data from an independent source with a video signal so as to allow simultaneous transmission of the two types of information without seriously degrading the picture quality. These systems generally rely on two properties: first, that a good deal of the information content of an unprocessed video signal is redundant and need not be encoded, and second, that intentional distortions of a video signal can be tolerated by the visual integration performed by the observer, provided that the site and amount of distortion are judiciously selected. Examples of such prior art systems are described in U.S. Pat. No. 2,266,194 issued to G. Guanella on Dec. 16, 1941 and patent 3,456,071 issued to R. N. Jackson et al on July 15, 1969, both of which provide for time multiplexing of a visually imperceptible supplementary signal together with a video signal. In accordance with the technique described by Jackson and Guanella, the added data totally replaces the video information at the locations at which it is inserted. When the video signal is reconstructed, a certain amount of resolution is therefore lost, and this factor must be taken into account in the tradeoff between the amount of supplementary data that can be added and the permissible picture degradation.
Other techniques that have been used to combine supplementary data with a video signal include inserting the data in the horizontal flyback interval, which is a form of time division multiplexing, or the more conventional use of TDM wherein the video signal is converted to digital form, compressed using any one of numerous compression strategies, and wherein the supplementary information is then inserted in the time slots made available by compression. These techniques, which require transmission of data in digital form, suffer in that the error tolerance of the transmission medium is smaller than for analog transmission, and costly error correcting circuits may be required. Frequency division multiplexing, which is another alternative, requires filtering which can also be expensive and complex.
In view of the foregoing, it is the broad object of the present invention to enable transmission of a supplementary digital signal together with a video signal, without unduly distorting the latter. The encoding is to be accomplished so that analog transmission is possible. However, some of the internal processing within the encoder may be digital or analog, as desired.